The instant invention is directed to a method of producing coarse-crystalline nicotinic acid with a high degree of purity which is precipitated from alkaline alkali nicotinate solutions by the addition of mineral acid.
Many methods of producing nicotinic acid are known.
It can be obtained by liquid phase oxidation of alkyl pyridines (German Pat. No. 2,046,556; Masuda U.S. Pat. No. 4,001,257) acid hydrolysis of 3-cyanopyridine (German Pat. No. 2,438,263) or saponification of 3-cyanopyridine with ammonia and subsequent decomposition of the ammonium nicotinate by water vapor distillation. The use of sodium hydroxide solution for this purpose is known (C.A. 65, 5446 a 1966; Polish Pat. No. 50,077); however, the authors of this patent consider ammonia to be more suitable.
German Pat. No. 828,246 is also directed to the saponification of 3-cyanopyridine. According to this patent, the 3-cyanopyridine is caused to react with an excess of barium hydroxide, then the barium is precipitated with sulfuric acid and a raw nicotinic acid is separated out which must still be purified.
The low selectivity has up to now prevented an industrial realization of the catalytic gas phase reaction.
The methods which obtain nicotinic acid by the ammoniacal saponification of 3-cyanopyridine have the disadvantage that the nicotinic acid amide (nicotinamide) byproduct which accumulates here reduces the selectivity and the yield and, in addition, long reaction times are necessary (J. E. Paustian et al. Chemtech, Mar. 1981, 176 FIG. 4; J. A. Arkhipova et al. Zurnal Prikl, Khim. 35 (2), 366-369, 1962; B. V. Sovorov et al. Zurnal Prikl, Khim. 45, 2716-2718 (1972). Due to the strong tendency to sublimation of the nicotinic acid, the thermal decomposition of ammonium nicotinate by water vapor results in operational problems as a consequence of cloggings which can only be minimized by expensive apparatuses (German Pat. No. 2,423,755).
A disadvantage of all the methods described above is the fact that a nicotinic acid is obtained which does not meet the qualitative requirements for pharmaceuticals and foods until after additional purifying measures (Ullmann, Encyklopadie der technischen Chemie, 4th edition. volume 23, page 709 (1983).
In addition, the product accumulates in a finely crystalline form and is very dusty. The particle size distribution extends from 5.mu. to 150.mu.. Since nicotinic acid is very irritating to the skin (Kirk-Othmer, Encyclopedia of Chemical Technology, volume 24, 3d edition, page 86), this can cause hygienic problems in the work area both for the manufacturer and also during further processing.
One tries to avoid this problem by granulating the finely crystalline nicotinic acid and thus obtaining a product with a particle spectrum between 100 and 315.mu. (min. 90%) (Lonza Revue 1/85, 14015).
The invention has the task of creating a method which makes it possible to produce coarse-crystalline nicotinic acid suitable for pharmaceuticals and foods without granulation and purification steps by means of the saponification of 3-cyanopyridine with alkali hydroxide and subsequent precipitation of nicotinic acid with high yields.